What is the Shar'i (Islamic legal) limit for expiation for accidentally killing my younger sister after I threw a blanket on her while she was an infant, leading to her death by suffocation?

What happened on your part is accidental killing. There is no difference in it between a minor and an adult, and Allah has pardoned unintentional errors. The obligation in this case is blood money (Diyyah) to your sister's heirs, and it falls upon your 'Aqilah (your paternal relatives from the 'Asabah). You do not inherit anything from her. And expiation (kaffarah) is obligatory upon you from your own wealth, which is the freeing of a believing slave. If you do not find one, then you must fast for two consecutive months.